Spin waves (SW) possess specific properties that make them well-suited for microwave applications. Very often these applications rely on magnetic bodies where large populations of coherently pumped spin waves share a common space with their less coherent biproducts and with the thermal populations of spin waves. It is thus of interest to develop experimental techniques able of measuring spin waves in broad frequency intervals with a large dynamic range, ideally from the floor of the thermal population of spin waves up to the regime of large amplitudes of magnetization precession. We have studied how a population of spin waves can be characterized from the analysis of the electrical microwave noise delivered by an inductive antenna placed in its vicinity 1. The measurements are conducted on a synthetic antiferromagnetic thin stripe covered by a micron-sized antenna that feeds a spectrum analyzer after amplification. The antenna noise contains two contributions. (i) The population of incoherent spin waves generates a fluctuating field that is sensed by the antenna: this is the “magnon noise.” (ii) The antenna noise also contains the contribution of the electronic fluctuations: the Johnson-Nyquist noise. The latter depends on all impedances within the measurement circuit, which includes the antenna self-inductance. As a result, the electronic noise contains information about the magnetic susceptibility of the stripe, though it does not inform on the absolute amplitude of the magnetic fluctuations.

For micrometer-sized systems at thermal equilibrium, the electronic noise dominates and can account for the measured entire noise (see Figure 1) ; in this case the pure magnon noise cannot be determined. If in contrast the spin wave bath is not at thermal equilibrium with the measurement circuit, and if the spin wave population can be changed then one could measure a mode-resolved effective magnon temperature provided specific precautions are implemented.

References
1 T. Devolder et al., Phys. Rev. B 105, 214404 (2022)